Kadarkaraisamy Mariappan

Selective Fluorescence Sensing of Copper(II) and Water via Competing Imine Hydrolysis and Alcohol Oxidation Pathways Sensitive to Water Content in Aqueous Acetonitrile Mixtures

Addition of hydrazines to a 1,8-disubstituted anthraquinone macrocycle containing a polyether ring produces site-selective imination, where hydrazone formation produces the more sterically hindered adduct. Reduction of the remaining carbonyl group to a secondary alcohol followed by addition of copper(II) ion causes intense yellow fluorescence to occur, which is selective for this metal cation and allows this system to be used as a fluorescence sensor. In the presence of water, a green-fluorescent intermediate appears, which slowly decomposes to produce the original starting anthraquinone. The addition of a large amount of water radically changes the reaction pathway. In this case, oxidation of the secondary alcohol is kinetically faster than hydrolysis of the hydrazone, although the same anthraquinone product is ultimately produced. Stern-Volmer data suggest that dioxygen quenches the green emission through both dynamic and static mechanisms; the static ground-state effect is most likely due to association of oxygen with the copper-bound fluorescent intermediate.

The chemistry of constrained crown ring systems and fluorescence sensor applications

Using anthraquinone as a useful synthetic scaffold and the ability of anthraquinone to form stable intermediate reduction products (i.e. anthrones and anthranols), we have synthesized a wide variety of constrained crown ring systems where the receptor includes several types and patterns of Lewis bases that can tune receptor selectivity for different metal cations. Constrained crown ring systems are defined as macrocycles that contain an intraannular heteroatom, in addition to the normal peripheral Lewis bases that compose the outer ring of the macrocycle. These fluorescence sensors predominantly utilizes the internal charge transfer mechanism to promote fluorescence, but has also led to the development of new photophysical mechanisms, i.e. metal-mediated tautomerization, to selectively detect Zn(II) ion in solution. We are currently pursuing a number of synthetic avenues to incorporate new functional groups and lumophores such that a myiad of different photophysical mechanisms under optimal conditions can be employed to improve solubility, sensitivity and take advantage of the cross pollination of electrochemistry and fluorescence spectroscopy with these sensors which incorporate closely integrated electrochemical, fluorescence and receptor subunits.

A new detection mechanism involving keto–enol tautomerization: selective fluorescence detection of Al(III) by dehydration of secondary alcohols in mixed DMSO/aqueous media

A new mechanism for the fluorescence detection of metal cations in solution is introduced involving a unique keto–enol tautomerization. Reduction of 1,8-anthraquinone-18-crown-5 yields the doubly reduced secondary alcohol, 2. Compound 2 acts as a chemodosimeter for Al(III) ions producing a strong blue emission due to the formation of the anthracene fluorophore, 3, via dehydration of the internal secondary alcohol in DMSO/aqueous solution. The enol form is not the most thermodynamically stable form under these conditions however and slowly converts to the keto form 4. Reduction of 1 with Fe/AcOH or the reaction of 2 with HCl directly yields compound 4, the keto tautomer of 3, which also produces the same blue emission in more polar solvents. Competition studies reveal that compound 2 produces a blue emission exclusively in the presence of the strong Lewis acidic Al(III) ion and at relatively low pH.